This is from Gueranger's reflection on the Ember Days in Advent, in The Liturgical Year.
Today the Church begins the fast of Quatuor Tempora, or, as we call it, of Ember days: it includes also the Friday and Saturday of this same week. This observance is not peculiar to the Advent liturgy; it is one which has been fixed for each of the four seasons of the ecclesiastical year. We may consider it as one of those practices which the Church took from the Synagogue; for the prophet Zacharias speaks of the fasts of the fourth, fifth, seventh, and tenth months. Its introduction into the Christian Church would seem to have been made in the apostolic times; such, at least, is the opinion of St. Leo, of St. Isidore of Seville, of Rabanus Maurus, and of several other ancient Christian writers. It is remarkable, on the other hand, that the orientals do not observe this fast.
From the first ages the Quatuor Tempora were kept, in the Roman Church, at the same time of the year as at present. As to the expression, which is not infrequently used in the early writers, of the three times and not the four, we must remember that in the spring, these days always come in the first week of Lent, a period already consecrated to the most rigorous fasting and abstinence, and that consequently they could add nothing to the penitential exercises of that portion of the year.
The intentions, which the Church has in the fast of the Ember days, are the same as those of the Synagogue; namely, to consecrate to God by penance the four seasons of the year. The Ember days of Advent are known, in ecclesiastical antiquity, as the fast of the tenth month; and St. Leo, in one of his sermons on this fast, of which the Church has inserted a passage in the second nocturn of the third Sunday of Advent, tells us that a special fast was fixed for this time of the year, because the fruits of the earth had then all been gathered in, and that it behoved Christians to testify their gratitude to God by a sacrifice of abstinence, thus rendering themselves more worthy to approach to God, the more they were detached from the love of created things. "For fasting," adds the holy doctor, "has ever been the nourishment of virtue. Abstinence is the source of chaste thoughts, of wise resolutions, and of salutary counsel. By voluntary mortification, the flesh dies to its concupiscence, and the spirit is renewed in virtue. But since fasting alone is not sufficient whereby to secure the soul's salvation, let us add to it works of mercy towards the poor. Let us make that whihc we retrench from indulgence, serve unto the exercise of virtue. Let the abstinence of him that fasts, become the meal of the poor man."
Let us, the children of the Church practice what is in our power of these admonitions; and since the actual discipline of Advent is so very mild, let us be so much the more fervent in fulfilling the precept of the fast of the Ember days. By these few exercises which are now required of us, let us keep up within ourselves the zeal of our forefathers for this holy season of Advent. We must never forget that although the interior preparation is what is absolutely essential for our profiting by the coming of our Lord Jesus Christ, yet this preparation could scarcely be real unless it manifested itself by the exterior practices of religion and penance.
The fast of the Ember days has another object besides that of consecrating the four seasons of the year to God by an act of penance: it has also in view the ordination of the ministers of the Church, which takes place on the Saturday, and of which notice was formerly given to the people during the Mass of the Wednesday. In the Roman Church, the ordination held in the month of December was, for a long time, the most solemn of all; and it would appear, from teh ancient chronicles of the Popes, that, excepting very extraordinary cases, the tenth month was, for several ages, the only time for conferring Holy Orders in Rome. The faithful should unite with the Church in this her intention, and offer to God their fasting and abstinence for the purpose of obtaining worthy ministers of the word and of the Sacraments, and true pastors of the people.